Causes and consequences of urban-associated song variation: A study of vocal behavior in the northern cardinal (Cardinalis cardinalis)
Desiree L. Narango, MS
Advisor: Amanda Rodewald
Animal communication systems, which rely upon complex cognitive behavior, specific social contexts, and environments that permit effective transmission, are vulnerable to disruption by anthropogenic disturbance. Forests in urban landscapes are known to differ from rural forests in terms of invasive exotic shrubs, avian communities, and anthropogenic noise. Although these urban-associated differences can elicit demographic consequences, little is known about the sub-lethal behavioral effects. Recent studies have implicated anthropogenic noise as a cause of changing bird song in urban areas; however, few have considered alternative explanations, nor the evolutionary and ecological consequences of altered songs. I investigated song variation in an urban landscape by asking the following questions: 1) How do the structural and behavioral components of bird song change across a rural-urban landscape gradient? 2) Which aspects of urbanization (e.g., noise pollution, invasive plants, avian community and morphology) best predict changes in song properties? and 3) Does urbanization alter relationships among song, indicators of fitness (e.g., reproductive output), and male quality (e.g., morphology, parental care and territory quality)? I investigated these questions by recording vocal behavior and monitoring the breeding activity of 54 individually-marked male Northern Cardinals (Cardinalis cardinalis) at nine sites distributed across riparian forests within a rural to urban landscape gradient in central Ohio in 2011.
Forests within urban versus rural landscapes differed ecologically so that urban forests had greater densities of conspecifics, denser understory vegetation with greater numbers of large trees, louder ambient noise (primarily from traffic), and smaller birds than more rural forests. As expected, cardinal song also changed with urbanization, with songs becoming longer, faster and with higher minimum, maximum and peak frequencies (Hz) as urbanization increased. Ambient noise at the territory level explained shifts in minimum frequency, whereas changes in conspecific densities best explained temporal variation in song structure (e.g., song length and syllable rate). Peak and maximum frequencies were not well explained by habitat or morphological models and instead may be driven by factors not considered in this study, such as cultural dialects.
Relationships between song and indicators of fitness and quality were not consistent across landscapes for some, but not all, song characteristics. In rural landscapes, the largest birds sang songs that were short and slow; however, song of urban males was not associated with body size. The lack of an association between song and body size in urban birds may be due to increased interspecific aggression from the aggregation of high numbers of territorial males. Thus urbanization may weaken the usefulness of temporal aspects of song as a reliable indicator of size. Relationships between song and parental care were consistent across landscapes, where birds that sang short, slow songs provisioned less and had nestlings in poorer condition. Males that used higher minimum frequencies also had nestlings in poor condition, a possible effect of habitat quality in noisy territories. Male song was not related to reproductive output or territory quality in either landscape.
To my knowledge, this is the first study to simultaneously evaluate the evidence for multiple potential mechanisms of urban-associated changes in song, as well as investigate consequences for signal reliability and fitness. These results suggest that a variety of factors, not anthropogenic noise alone, influence song in urban birds and these changes can affect signal reliability in novel environments.